Some random thoughts....
Vinegar in hard water can sometimes make ammonia issues worse.
I've been amazed by washing soda... Try adding a tablespoon of that instead of the oxi.
You're putting a lot into the diapers that might not all be rinsing out.
Maybe use a diaper sprayer to rinse diapers immediately after use.
Lanolin is an amazing barrier because it breaks down urine and it is most similar to skins natural oils.
Consider whether you are over cleaning his skin. Even hospitals recommend washcloths with water for wipes unless there's a poop. Skin needs its natural barrier.
Stuff that can irritate skin.... Aloe (sometimes it helps sometimes not), fragrances, detergent residue, essential oils, products with food type ingredients, combining too much warmth with too little air (try a breathable cover)...
Herb Pharm Trauma Oil has olive oil, calendula for healing, hypericum for pain, and Arnica. Works great for us mixed with Lanisoh.